ad: dxeng

Many 4 foot ground rods vs one 8 foot?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NZ9Y, Oct 15, 2011.

Thread Status:
Not open for further replies.
  1. NZ9Y

    NZ9Y Premium Subscriber

    I suspect the single 8 foot rod is better. Am I right? Or would several 4 footers some close distance together be acceptable? If so, how many and how close? If we need a soil composition, assume average to above average conductivity.

    Just wondering.
     
  2. NN4RH

    NN4RH Subscriber

    For what?

    If it's for your house AC power ground, If you use rods for grounding electrodes, 8 foot (at least) is what's specified by NEC.

    If it's for lightning protection, I don't know. My gut feeling is that it isn't the total length of rod that matters but the volume of earth around the ground rod that can dissipate energy. And I doubt if it adds up linearly. In other words, two 4-foot rods probably won't equal one 8-foot rod. If it did, everyone would be using 4-foot rods! Easier to drive in.

    If it's for RF ground for something, ground rods don't really do anything for you.
     
  3. K9KJM

    K9KJM Ham Member

    Question is why? Are you unable to drive an 8 foot rod all the way in because of bedrock? (A very common problem around here)

    If that is the case, The standard 1/2" or 5/8" diameter copperclad 8 foot rods can be cut in half and more driven in. Rods are spaced about twice the distance apart as the depth. (8 foot rods are spaced about 16 feet apart, 4 foot deep rods should be spaced about 8 feet apart, etc)

    It is almost always better to have fewer deeper rods than more shorter rods.

    NEVER use the big box store bought 3/8" diameter 4 foot rods that are simply thin copper PLATED, Not copperclad! Those turn to rust in about one year or less.
     
  4. N0AZZ

    N0AZZ Ham Member

    Always use the 5/8"x8' copper clad rods buy them from your local electrical supply house not a big box store they are quite different in price the copper clad is so thin on the cheapest they aren't worth buying.

    If you have trouble driving them borrow a hammer drill and a ground rod bit and use it I have one but it seems friends have it most of the time, but that's why I bought it.
     
  5. NZ9Y

    NZ9Y Premium Subscriber

    I guess I forgot to say what it was for. :) Lightning. I will have to drive rods again soon at the base of a 50' tower and was debating between one 8' or four 4' rods. I just remembered they will also act as an RF ground for a sloper, so I'm leaning toward the 4.

    I know someone is going to say "just drive all four 8' rods". Well I just cant. I physically do not have it in me until maybe spring and I need this done for winter. And the economy must be booming here in MN because I cant get anyone to help me even when waving handfuls of $100 bills. Today, I'll be attempting to dig a 4x4x4 hole while on crutches. Yay!
     
  6. N4EYZ

    N4EYZ Ham Member

    In the commercial tower business we always prefer many, many ground rods. (lightning protection) I'm not a grounding protection authority, I'm just telling you what all the big players do. People like Motorola, Verizon, AT&T and so on.

    We installed 10' rods spaced every 10' in a ring around the tower and then again around the equipment shelter. The rods were then joined by typically #2 or 4/0 copper(difference depending on customer's spec's) All connections are cad-welded (exothermic welding). They permitted no mechanical connections below ground (split bolt or clamp type).

    When it came to rocky soil we just drove them in as far as possible and then moved to the next rod. (all of this is done in a 18 to 24" deep trench and then back-filled when done)

    In your case I would prefer four 4' rods to one 8'. If you have no cad-welding experience (most don't) don't sweat it. It's a tad pricey and potentially dangerous without experience. Just make sure all connections are clean and shiny and use good quality clamps when joining rods and wire. (use the largest wire you can afford. Typically #2 stranded is great and easy to handle. Take one lead off each tower leg to your ground ring.

    Wayne
     
  7. K9STH

    K9STH Ham Member

    For lightning protection each tower leg should have its own ground rod with the lead from each leg going through a gentle curve. Lightning definitely does not like sharp angles in the conductor and will almost always jump from the conductor if the bend radius is too sharp. If possible, the ground rod needs to be 8 feet long.

    For r.f. grounding things are somewhat different. In most soil types the maximum effective grounding takes place in the first 5 feet and extensive experiments have shown that when the rods are spaced 2.4 times the length of the ground rod apart this is maximized. Of course, the old "rule of thumb" of spacing the rods twice their length apart works fine, it is just that this spacing does not optimize the effective grounding. Using the 2.4 spacing then 2 each 5 foot ground rods spaced 12 feet apart will approximately double the effective grounding of a single 10 foot ground rod. If 4 foot rods are used, then spacing them 9.6 feet apart will increase the effective grounding by about 1.6 times that of a single 8 foot ground rod.

    The calculations behind the 4 foot ground rods versus an 8 foot ground rod are as follows: Most of the effective grounding takes place in the first 5 feet and therefore, for all practical purposes, the extra 3 feet of the 8 foot ground rod is contributing very little. Comparing a 4 foot rod with the 5 feet means that the 4 foot rod will have approximately 80 percent of the effective grounding of the 5 foot rod. Then, installing 2 each 4 foot rods 9.6 feet apart will have about twice the effective grounding of a single 4 foot rod. Multiplying the 80 percent by 2 gets 160 percent or 1.6 times the effective grounding by installing 2 each 4 foot ground rods versus a single 8 foot ground rod. Of course, the actual calculations are more involved. However, the error introduced into the calculations by simplifying the parameters is only a very small percentage.

    Glen, K9STH
     
  8. W8JI

    W8JI Ham Member

    Unless ridiculously short in length, more rods spread out are always much better than one deeper rod for lighting or RF.

    Four foot deep in medium or good soil is almost the same as eight foot deep for lightning. We are almost always better off with two properly spaced 4-foot rods than a single 8 footer if the soil is normal moisture and conductivity. This not true for power line frequency or DC ground resistance, but it is for RF and lightning.

    None of my towers have deep rods. They have buried radials and many shorter rods. My house power mains ground is deep, because that is a code requirement.

    http://www.w8ji.com/station_ground.htm

    For lightning protection, how you route wiring and bond grounds and entrance panels together is far more important than anything else, including grounds!!!
     
  9. N8CPA

    N8CPA Premium Subscriber

    I still have to install ground rods for my station. My dream of dreams is to rent a TE-905 ground rod system. Take a look at it on Youtube. You attach an adjustable vise to the ground rod at a comfortable height. Then you drive that with the hammerdrill. When it reaches the surface, you slide it up the ground rod, retighten and drive some more. You finish a typical ground rod attachment.

    And it looks like Erico has a similar muscle powered system.
     
  10. NZ9Y

    NZ9Y Premium Subscriber

    SO I would imagine radials plus ground rods would be ideal for lightning and RF.

    The job is a basic triangular tower less than 50' high. There are many taller structures in the area. I will have a tribander on top, MAYBE a 2 meter vertical, and am planning to run a 160-40 meter sloper.

    I know the sloper is looking for a good RF ground. My plan was to do three 4 foot rods for each tower leg, running #4ish copper in a gentle arc from several feet up the tower to each rod. One run of wire will run the entire length of the tower from the mast to one of the rods. Then I would bond all the rods together.

    As I excavate, I will be ripping up wire from a radial field nearby. That few dozen yards of wire I will bond to the rods as well and spread out(I may as well, it cant hurt and its there). I will then do lightning arrestors on each of the 2 (maybe 3) coax runs at the bottom of the tower and run them to the closest rod on thier route to the shack.


    I know lots of guys do alot less, and others do more. If I do this (and all the stuff at the house end too), can I call it good? Any MINOR changes to this plan I should consider?
     
  11. KA0GKT

    KA0GKT Ham Member

    A local to lrental store will gladly rent you a roto-hammer drill and a groundrod driving bit. I used one rented from Sunstates to drive eight ground rods into the Coleche at an AM transmitter site as the grounding points for a transmission line entrance port.. I further silver brazed 2" copper strip to each rod and up to the ground bus as low resistance connections are a must when providing lightning protection for 3-1/8" rigid transmission line.

    Check out a local United Rental...perhaps even Menards has a rental department.
     
  12. K4SAV

    K4SAV Ham Member

    The sloper will work best if it never sees a ground. Unfortunately on 160 meters that will never happen on a 50 ft tower with a tribander. You need a much taller tower and much more top loading to make this antenna work well on 160. The best you could do would be to add a radial system at the base of the tower. Even then it won't be very good on 160. It's almost exactly equivalent to putting up an inverted vee with a very acute included angle and tying one end of the vee to a ground rod. No one would do that. However it might have acceptable performance on 80 meters. Getting a multi-band sloper to work is very difficult. The best attachment point to the tower varies depending on the band.

    I always recommend at least 9 ground rods at the tower, three for each leg spaced twice their length apart. I recommend that because I know that stands up to direct lightning hits. I also know that only one ground rod is almost equivalent to no ground rods at all and usually results in lots of damage. In between those, there might be something that works but I haven't tested it. There is lots of variability depending on how you connect the rest of the system together and how far the tower is from the house.

    Jerry, K4SAV
     
  13. NA0AA

    NA0AA Ham Member

    If you don't want to use power tools, a slide hammer, commonly used to drive T-posts, can do the hardest part of driving ground rods - I use it to get them down to about the last 2-3', then use a sledge hammer for the final few feet - if you dig a small hole in which to drive the ground rod, you can get even farther with the hammer.

    For a permanent attachement, Cadwelds are awsome, and fun to do, but they are not easy for amateurs to install properly. Probably the easiest would be to use MAPP gas torch and 5% or 10% silver solder - clamp the conductors, then use the silver solder on top of this - the silver solder will take ground contact, regular lead/tin crumbles in short order.

    Unfortunately, the silver stuff has gotten a touch pricy lately - not suprise given price of silver. IIRC, Georgia Copper used to sell individual sticks, most supply stores only sell by pound!
     
  14. NZ9Y

    NZ9Y Premium Subscriber

    I was looking at a sloper from Alpha Delta. They pretty much say the opposite about grounding a sloper. The "sloped" wire is one side and the well grounded tower is the other.


    It will probably work better on 160 than what I have now, which is nothing. I simply dont have the space to do a loop or dipole on 160 or 80. A shortened sloper is the best I can do with the space I have. I'm sure something closer to a commercial AM broadcast tower would work better. :)


    Wow! I haven't seen all that many amateur towers, but I dont remember anything that elaborate. I didn't see anything like that in the tower install docs either. Its a wonder anyone ever puts up a tower at all!

    I'm not trying to be obtuse, but I want to be sure I understand. A 'lil old 40 foot tower holding a tribander and sloper, with all sections bonded with 4 guage copper to a rod for each leg and lightning arrestors on each coax at the base to one of the rods is woefully inadequate.

    You're pulling my chain right? Most hams have 9 or more ground rods driven around thier towers?
     
  15. W8JI

    W8JI Ham Member

    That's more than a little incomplete.

    The slopper is like a dipole cut in half, with the other half of the dipole the tower and everything on and connected to the tower. There is exactly as much current leaving the feedpoint of the slopper into the tower as there is into the sloping wire you call the antenna. Everything conductive on the tower, connected to the tower, and in close proximity to the tower all is part of the radiating system.

    With that in mind, and because the base is 40 feet away, and because the rods just will never be a good RF ground, grounding at the base with a few rods is about useless for affecting RF performance. The height of the tower and what is on the tower has far more effect.
    Just install the antenna and see if it works. If you have a tribander on the top, and maybe some metallic guy lines attached to the tower, it will probably work.
    It doesn't matter what is on the tower. A ground is a ground. A single rod on each leg isn't much of a ground at all, especially for RF or lighting. Frankly though, the last thing I would worry about with the sloper is a good RF ground 40 feet away at the tower base.
    I don't know about other people, but I have about 4 or more short ground rods on all my towers. Nearly all of my "tower ground" comes from buried bare radials that are 50-200 feet long depending on the tower. I have stone around the pads, and at the edge of the stone I have a heavy buss wire or copper flashing that connects between the 4 or more rods in a square shape. Off that heavy wire or flashing, I have many buried radials.

    That's all the ground needed. My towers are very tall, the tallest things by far for miles, so they get hit all the time. Never a problem.

    73 Tom
     
  16. NZ9Y

    NZ9Y Premium Subscriber

    Maybe I opened this can of worms by being unclear. When I was thinking lightning, I was thinking some protection from pulses. I hadn't planned to build a system that could take repeated direct hits while I merrily ragchew in safety.

    I have neighbors with metal chimneys that are taller than my little tower will be. I'm pretty sure they dont have 9 rods and a radial field.

    If a storm nears, I'll disconnect. If I forget and the tower takes a direct hit, I imagine my radio will die.

    I just want to know my house will be safe. If I run rods at the tower, before the house, have a bus in the shack and bond them all together and to the house ground, have I done it right?
     
  17. K9KJM

    K9KJM Ham Member

    Forget about running a copper wire all the way from the tower base up to that mast. That is a WASTE of copper wire! Use that same wire in the ground where it will do some good. Read the Polyphaser notes and see how they have proven that running a wire up a tower is actually more than just a waste of time, It can cause problems.
    While all bends do need to be gradual as pointed out, You do not need to run the ground wires a "few feet" up the tower. Just think of the bend or radius of the wire to be about like bending it around a 5 gallon pail....... Running from ground level would put it about 8 or so inches above ground level.
    IF you decide to "disconnect" things (Actually a very dangerous practice) Be SURE to have some way to properly ground those disconnected lines!

    For some tips on how to do it on a low budget:

    http://www.scribd.com/doc/14868226/lightning-protectiontaming-thors-thunderon-a-budget

    Give that site plenty of time to load.
     
  18. K4SAV

    K4SAV Ham Member

    Manufacturers say all kinds of things about the stuff they are selling. For a sloper to work well, most of the currents should be confined to the sloping wire and the top of the tower, with very little current flowing down the tower. Most of the current down the tower will be dissipated in the ground, and it also decreases the feedpoint impedance because it cancels some of the radiation from the sloping wire. A 50 ft tower will not look like a high impedance at 160 meters so large amounts of current will go down the tower. It will radiate something and you can probably make some contacts with it. It just will not be very good.

    True.

    I haven't taken a survey to see what the average ham does. I know lots of people have one ground rod and many more have nothing. They have to learn the hard way. I have seen what lightning does with one ground rod. It essentially ignores it and looks for the real ground (whatever is close). I know many hams have many more than 9 ground rods. I think K8RI has 30 something. That could be an overkill. The Polyphaser book "The Grounds for Lightning Protection" shows from 9 to 21 ground rods on their diagrams. I recommended what I think will work. I know it works for me and I don't consider it an overkill. I would feel very uncomfortable with less. My tower gets a direct hit about 2 times per year (4 this year) and no damage so far.

    Notice in W8JI's post he has a different grounding system, but what he uses is orders of magnitude better than one ground rod.

    One ground rod should be sufficient for that, provided the strikes are not too close.

    If your grounding is poor enough that your radio dies, it is likely that a lot more stuff in the house will die also. If the tower is right next to the house and only has one ground rod, it is likely that the house slab, chimney, water pipes, or wiring will be one of the main terminal points for the strike, because lightning won't think that one ground rod really looks like ground. Lightning likes slab and rebar in concrete and will go thru lots of stuff to get to it.

    There are all sorts of ways to mess it up. A short connection between radio entrance panel ground and the AC electrical ground is of major importance. You want many ground rods at the tower to dump most of the current into the ground before it reaches the house. The further that current has to travel before it reaches the house, the less current will be on the wire to the house. The bus in the shack has very little effect on lightning protection (contrary to what most hams think). If you have lightning currents on that bus, your system is not configured properly and probably will sustain major damage with a strike.

    Jerry, K4SAV
     
  19. KM1H

    KM1H Ham Member

    I dont know anyone who has more than one per leg which was considered overkill not too long ago. This recent obsession appears to be driven by those selling something thru magazines, Internet, etc.

    Its amazing that so many installations of decades past never had a problem with one 8' rod driven in whatever way it took to get most of it in some sort of soil.

    My 180' tower (was 160' for the first 10 years) on top of the highest hill in 20+ miles has survived for 22 years and so has all the ham equipment in the house. The only damage Ive ever found is several P&B 24VDC power relays used for yagi stack switching that now only work on AC.

    Carl
     
  20. K4SAV

    K4SAV Ham Member

    Not so amazing. I might add, for the first 4 years of my ham career (1950's), my antennas never had a ground of any kind. I never lost any equipment to lightning those 4 years either.
     
Thread Status:
Not open for further replies.

Share This Page